The present invention relates to a process and apparatus for determining the ratio of components in a fluid mixture, particularly a sensor for use in determining the ratio of water to methanol in a reforming feed for producing hydrogen for supply to a fuel cell system.
The use of fuel cells in electrically powered vehicles has become important for the purpose of reducing air pollution normally resulting from internal combustion engines. As a way of producing the hydrogen gas to be supplied to the fuel cell in combination with oxygen gas, usually supplied as the oxygen present in atmospheric air, it is known to supply a mixture of methanol and water to a catalytic reforming reactor to convert said mixture to carbon dioxide and hydrogen according to the endothermic reaction:
CH3OH+H2Oxe2x86x92CO2+3H2.
Such a system is described in the U.S. Department of Energy Report (DOE/CH/10435-02) of January 1996, entitled Research and Development of Proton-Exchange-Membrane (PEM) Fuel Cell System for Transportation Applications.
In order to operate such a reforming system it is important that the ratio of methanol to water in the reforming feed be continuously monitored and correspondingly controlled so that the optimum ratio of these reactants for the reforming reaction is achieved. The measurement of the ratio of methanol to water can be carried out by chromatography methods. However, such techniques used in the laboratory are not practical for industrial or commercial applications, such as for fuel cell systems in electrically powered vehicles or in electric power sources suitable for other purposes, such as in satellite communication devices and in stationary xe2x80x9chomexe2x80x9d power systems. In general it is desirable to have a sensor suitable for continuously measuring the ratio of components in fluids, both gases and liquids. Although it is useful to have a sensor measure the ratio of the methanol and water in the gas phase fed to the above reforming system, it is practical to measure the ratio of the methanol to water in the liquid phase in equilibrium with the gas phase mixture fed to the reformer.
It is known to use oscillometry for determining the water content of a liquid mixture since water has a dielectric constant fifteen to twenty times that of other substances (see PERRY, CHEMICAL ENGINEERS"" HANDBOOK, FIFTH EDITION, 1969, PAGE 22-52). LANGE""S HANDBOOK, THIRTEENTH EDITION, 1985, cited below, discloses dielectric constants for a number of substances, including water and methanol.
It is an object of the present invention to provide a process for determining the ratio of methanol to water in a fluid mixture containing same.
Another object of the invention is to provide a sensor for determining the ratio of two components in a fluid mixture, particularly a liquid mixture, mixtures with different ratios of said components having respectively different dielectric constants at given conditions of frequency of an imposed alternating voltage and of temperature of said fluid mixture.
It is a further object of the invention to provide a sensor for determining the ratio of two gas components in a gas mixture, mixtures with different ratios of said components having respectively different dielectric constants at given conditions of frequency of an imposed alternating voltage and of temperature and pressure when said mixture is placed between opposed electrodes imposing said alternating voltage.
In accordance with one aspect of the present invention, there is provided a sensor for determining the ratio of two components in a test fluid mixture, mixtures with different ratios of said components having respectively different dielectric constants at a given frequency of an imposed alternating voltage and at a given temperature of said test fluid mixture when said test fluid mixture is subjected to said alternating voltage, said sensor comprising:
a first pair of opposed electrodes establishing there between a test cell, the opposed electrodes of said test cell being in contact with said test fluid mixture being tested, said test cell having a test cell impedance;
a reference cell established by a second pair of opposed electrodes having equivalent operating characteristics to those of the first pair of opposed electrodes, said reference cell containing a reference fluid mixture of the two components in a known ratio and being isolated from said test fluid mixture, said reference cell having a reference cell impedance;
an electrical circuit wherein one of the opposed electrodes of the first pair of electrodes is connected to one pole of a source of alternating voltage and one of the electrodes of the second pair of electrodes is connected to the other pole of the source of alternating voltage, and wherein the other of the opposed electrodes of the first and second pair of electrodes are connected together for serial flow between said pairs of electrodes of the test cell and reference cell, respectively; and
a voltage measuring device connected to said circuit for measuring the voltage drop across the test cell so as to determine the voltage drop across the test cell in relation to the voltage drop across the reference cell;
said relative voltage drops being used to determine said test cell impedance relative to said reference cell impedance and thus to determine the dielectric constant of the test fluid mixture relative to that of the reference fluid mixture at a given temperature of said test and reference fluid mixtures and at,a given voltage of the imposed alternating voltage, whereby the ratio of said components of the test fluid mixture to the ratio of the components of the reference fluid mixture is correlated with the respective dielectric constants of said test and reference fluid mixtures.
In a modification of the sensor of the invention, representing a preferred embodiment, the reference cell is replaced by a reference element including a capacitor connected to the same source of alternating voltage as the first pair of electrodes, said reference element having a known impedance corresponding to that of such a (hypothetical) reference cell containing the reference fluid mixture of two components of a known ratio and at a specific temperature. In this embodiment, there is provided a temperature measuring device for measuring the temperature of the mixture in the test cell and a voltage measuring device (connected to the electrical circuit in which the reference element replaces the reference cell) for measuring the voltage drop across the test cell electrodes and across the reference element of known impedance so as to compare the respective impedances and determine the ratio of the two fluid components of the mixture in the test cell on the basis of the impedance of the fluid mixture in the test cell and the temperature of said fluid mixture.
In accordance with another aspect of the present invention there is provided a process for testing a fluid mixture containing methanol and water in a given ratio so as to determine said ratio, which comprises:
establishing a test cell between opposed, electrodes contacting said fluid mixture, said fluid mixture being tested at a given temperature and said test cell being in open flow communication with said fluid mixture being tested;
imposing an alternating voltage of a given frequency across said electrodes;
measuring the dielectric response of the fluid mixture in said test cell at said temperature and frequency, and
determining the ratio of methanol to water in said mixture on the basis of the dielectric response thus measured.
In the usual practice of this above process the dielectric response of the test cell is measured in terms of the dielectric constant of the fluid mixture in said cell, the dielectric response of the fluid mixture being tested varying in an essentially linear and monotonic relation to the ratio of methanol to water in said mixture at a given temperature of said fluid.
The sensor provided by the present invention measures the dielectric response of the fluid mixture, such as a methanol/water mixture, in either liquid or vapor phases. The invention takes advantage of the properties of such fluid components to have a dielectric response, namely, dielectric constant, measured at low frequencies (corresponding to a static dielectric constant), which varies linearly and monotonically with the ratio of the concentration of one component to the concentration of the other component, usually expressed as the weight percent of one component to the total weight of both components. In particular, the present invention concerns determining the ratio of the concentration of methanol to the concentration of both methanol and water in a fluid mixture containing same, usually a fluid mixture consisting essentially of said components, preferably consisting of said components. In the usual practice of the invention the electrical circuitry of the sensor acts as a capacitance divider which allows measurement of the dielectric response of a fluid mixture of an unknown composition at any instant time relative to the dielectric response of a known composition, particularly a fluid mixture of methanol and water corresponding to a desired weight ratio of a specific value.
In the basic practice of the invention there is provided a reference cell which has opposed electrodes of equivalent operational characteristics, which may be termed xe2x80x9ccell geometryxe2x80x9d, as the electrodes of the test cell, so as to facilitate the evaluation of the voltage drop or other operational characteristics of the test cell by comparison to the reference cell. Typically, the reference cell, except for the absence of the openings to the fluid in the enclosure, is identical in construction to the test cell. As will be described below, the reference cell participates in extending the dynamic range of the measurement of the dielectric constant, namely the capacitance, of the fluid mixture in the test cell. The reference cell is closed or isolated with respect to the fluid being measured whereas the test or sample cell is in open communication with the fluid in the enclosure so as to be in contact with said fluid. In the case of a liquid mixture, the electrodes of the test cell are preferably totally immersed in the liquid so that the total surface of the electrodes is in contact with the liquid mixture so as to insure a consistent area of contact with the liquid mixture for each test. Similarly, the opposed electrodes of the reference cell are preferably totally immersed in the liquid mixture in the reference cell. In one embodiment, both the outer walls of the test cell and of the reference cell are made of plastic with the edges of the opposed electrodes mounted in said walls. The reference cell and the test cell are in side-to-side relationship and separated by a common electrode. The outer electrodes of both cells are preferably in direct contact with the fluid being tested. Since the reference cell tends to be at substantially the same temperature as that of the test cell by virtue of the fact that the test cell and reference cell are both in heat exchange contact with the fluid mixture through the common electrode and the outer electrodes (even though the reference cell is not in fluid communication with the fluid being tested in the enclosure), the only variable is the difference in ratio between the two components of the fluid in the reference cell and the ratio of said components in the test cell. The fluid mixture of the reference cell has a ratio of methanol to water corresponding to a specific value for the mixture being measured in the test cell. In this way by comparative electrical circuitry used in the present invention it is possible to monitor the ratio of the mixture in the test cell so as to take steps to ensure that the ratio of the components of the fluid in the test cell, communicating with the fluid being sampled, is as close as possible to the desired ratio in the reference cell. Because of the greater heat transfer between outer walls of the cells and the fluid in the enclosure when the fluid is a liquid as opposed to a gas, the practice of the invention using a reference cell is of greater effectiveness when the mixture being measured is a liquid.
Determining the dielectric response of both the reference cell and the test cell is carried out by measuring the capacitance of both cells. Because of the equivalent operating characteristics of both cells and the substantially equal temperatures of the respective liquid mixtures one can determine the capacitance of the test cell as compared to that of the reference cell by appropriate measurements using the circuitry as described in greater detail herein. The difference in phase of the alternating voltage at the test cell and the reference cell is correlated with respect to the known static dielectric constants of various ratios of the mixture of methanol and water, or other components, at given temperatures and alternating current frequencies. The frequency of the applied voltage and temperature of the fluid mixtures of both the reference cell and the test cell are essentially the same so that the monotonic correlation between the ratio of these components and the dielectric constants of the mixtures in the respective cells (as a function of the capacitances of the fluid mixtures determined for said cells) can be utilized to effect a calculation of the ratio of the fluid components in the test cell by reference to xe2x80x9clook-upxe2x80x9d tables, correlating the linear and monotonic relationship of the static dielectric constants with various known ratios of the components as described above.
One can determine the phase difference between the test cell and the reference cell by current low-cost ADC circuitry. Matters can be simplified by choosing an alternating current frequency above the inverse of the bulk dielectric relaxation time expected for the fluid mixture being tested, in which event the complex impedances of both the reference and test cells reduce to simple capacitance calculations so that the determination can simply be that of the amplitude of the capacitance measured. 100 KHz is presently contemplated. The applied voltage is typically in the order of 10 millivolts.
The electrodes of the test cell as well as the corresponding reference cell are usually in the form of parallel plates or coaxial cylinders.
In the practice of a process aspect of the present invention instead of a reference cell containing a mixture of methanol and water of a known ratio, usually a desired ratio, and at a specific temperature corresponding to the temperature of the fluid of the test cell, one can use a fixed and stable capacitor of given capacitance or impedance corresponding to such a (hypothetical) reference cell. Typically the capacitor corresponds to a reference cell which is identical to the test cell. According to this aspect, it is necessary to measure the temperature of the test cell and compare the impedance or capacitance of the test cell to that of the reference capacitor so as to determine the capacitance of the test cell. One then determines the ratio of methanol to water in the fluid mixture of the test cell on the basis of the capacitance thus determined (as converted to the dielectric constant) and the temperature of the mixture in the test cell. In this case the determination is based on a three-dimensional plot of dielectric constant for the fluid methanol/water mixture as a function of composition and temperature, as is further discussed below. Corresponding xe2x80x9clook-upxe2x80x9d tables containing these parameters as incorporated in suitable software can be utilized to carry out the computation of the mixture ratios corresponding to the two-dimensional and three-dimensional look-up tables noted above.
In utilizing the process and sensor of the present invention in determining the ratio of components in a gas mixture, one must take into account the pressure of the mixture in the test cell as compared to that in the reference cell since the dielectric constant of the gas mixture is also a function of the pressure of a gas. The pressure variation can be used to adjust the calculation of the ratio of the gas component by taking into account the known relationship in the LANGE handbook noted below. Since an actual reference cell, in which the pressure (and possibly the temperature) must also be measured, is generally not practical for the gas embodiment, the practice of the invention in which the fluid mixture being measured is a gas mixture is usually carried out by providing a reference element of given impedance, said impedance being equivalent to that of a (hypothetical) reference cell having equivalent operating characteristics to those of the test cell, said reference cell having a gas mixture of methanol and water of a known ratio and of a known temperature and pressure and being operated at the same frequency as that of the test cell. The practice of the invention then includes:
measuring the temperature and pressure of the gas mixture being tested,
measuring the impedance of the gas mixture in the test cell,
comparing the impedance of the test cell to that of the reference element so as to determine the capacitance of the test cell, and
determining the ratio of methanol to water in the test cell on the basis of the capacitance, temperature and pressure of the gas mixture in the test cell.